The present invention relates to a method for continuously annealing a metal, e.g. steel, strip of continuous length in a vertical annealing furnace. More particularly, the invention relates to a method for continuously annealing a metal strip by passing through a vertical annealing furnace as being guided by a sequence of rolls where meandering and buckling deformation of the running metal strip can effectively be prevented.
Annealing of a metal strip of continuous length is performed usually by passing through an annealing furnace comprising a heating zone, a soaking zone and a cooling zone connected in series and the metal strip runs through these zones in this order. In a vertical annealing furnace, each zone is provided with two sets of hearth rolls in the upper and lower parts with their axes substantially in parallel with each other and the running metal strip is guided alternately by one of the upper rolls and one of the lower rolls hanging thereon to move vertically up and down between the rolls and to be finally introduced into the next zone.
One of the problems in the operation of an annealing furnace of this type is meandering of the running metal strip as it runs contacting with and winding around each hearth roll. The meandering here implied is a phenomenon that the center line of the running metal strip deviates to the right or left from the axial center of the hearth roll by the shift of the metal strip in the transverse direction.
In order to prevent the running metal strip from meandering as mentioned above, each zone of a conventional vertical annealing furnace is provided with one or two steering rolls to control the positioning of the metal strip. This means is, however, not quite effective so that it is generally understood that complete prevention of meandering of the metal strip is rather a difficult matter.
The hearth roll in the annealing furnace of the above described type usually has a configuration tapered at both end portions or a crowned configuration having varying diameters largest at the center and decreasing continuously toward the ends with a purpose to decrease meandering of the running metal strip in contact therewith. The principle in this case is the same as in the power transmission mechanism with belt and pulley in which a crowned or end-tapered pulley prevents the belt from coming off the pulley. In the belt and pulley of this type for power transmission, the belt coming into contact with the pulley is urged toward the center of the pulley where the diameter thereof is the largest because the belt is moved together with the pulley while it is kept in contact with the pulley by means of the friction therebetween. Similarly, the metal strip hanging on and guided by the hearth roll of the above mentioned configuration is urged toward the center portion of the roll where the diameter is the largest.
As a consequence of the use of a hearth roll which is not of a straightly cylindrical form, the distribution of the tensile stress in the longitudinal direction of the metal strip cannot be uniform in the transverse direction. In particular, the non-uniformity of the tension in the metal strip is increased when the taper angle of an end-tapered roll or the surface curvature of a crowned roll is large, when the thermal expansion of the hearth roll is larger at the center portion than at the side portions due to the heat radiation within the furnace and when the tension in the longitudinal direction of the metal strip is increased. When the tensile stress in the longitudinal direction has a non-uniform distribution in the transverse direction, a compressive stress is produced within the metal strip as a consequence. Accordingly, buckling of the metal strip sometimes takes place when this compressive stress exceeds a certain value.
When the hearth roll has an end-tapered configuration, for example, the buckling first takes place at positions contacting with the shoulder portions of the roll or the boundary portions between the straightly cylindrical portion at the conically tapered portions of the roll. Such a phenomenon of buckling should be avoided by all means not only due to the degraded product quality but also due to possible breaking of the metal strip depending on the extent of buckling. Once such a break has taken place in the metal strip, the whole process is unavoidably in great troubles including damage to the annealing furnace per se.
The phenomenon of buckling in the metal strip can of course be obviated when the hearth roll is of a straightly cylindrical form without end-tapering or crowning. Such a measure, however, cannot be undertaken in consideration of the meandering of the running metal strip. Thus, it has been eagerly desired to develop a method for simultaneously preventing meandering of and buckling in the running metal strip.
In recent years, on the other hand, a great variety of grades are desired in the supply of cold-rolled steel strips not only in the dimensions but also in the strengths as a material. It would be extremely difficult in the continuous annealing furnace of metal strips of the above described type, however, to simultaneously preventing meandering of and buckling in the running metal strip for all kinds of cold-rolled steel strips of different dimensions and under different conditions of thermal treatment by a mere optimization of the configuration of the hearth roll alone or rather to design a hearth roll to meet such a requirement.